This invention relates to composite blades for use in fluid flow machines and, more particularly, to increasing the torsional stability of such blades through selective orientation of the composite blade filament reinforced laminates.
The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.
For many years attempts have been made to replace the relatively heavy, homogeneous metal blades and vanes of fluid flow machines such as gas turbine engine compressors with lighter composite materials. The primary effort in this direction has been toward the use of high strength, elongated filaments composited in a lightweight matrix. Early work involved glass fibers, and more recent efforts have been directed toward the utilization of boron, graphite and other synthetic filaments. These later materials have extremely high strength characteristics as well as high moduli of elasticity which contributes to the necessary stiffness of the compressor blades and vanes.
Many problems have confronted the efforts to utilize these filaments, particularly in adapting their unidirectional strength characteristics to a multidirectional stress field. To a large extent, these problems have been overcome and composite blades have been demonstrated with performance characteristics, in many areas, equal to or better than their homogeneous metal counterparts in addition to providing the expected and significant weight reductions.
However, several difficult design problems remain to be solved. In particular, composite blades generally exhibit a relatively low torsional frequency, implying that the torsional stability limit of the blades is relatively low. With such a low torsional stability limit the blade could conceivably be overstressed in torsion with subsequent torsional, vibration-induced, blade fatigue failure. As the blades for fluid-pumping machines have been designed to operate at increasingly higher speeds, such as experienced in gas turbine engine compressors and fans, the shape of the airfoil portion of the blade has migrated from a relatively thick, moderately cambered subsonic section to a relatively straight, low-cambered, supersonic section, thus tending to reduce the torsional stability limit even further.
One approach toward increasing the torsional stability of a composite blade involves the incorporation of a metallic core within the airfoil portion which serves as a torsional stiffener. Variations in material, size, shape, and disposition of the core will affect the torsional resonance of the blade. However, this approach has the disadvantages of having to bond the composite shell to the metallic core, the increased weight due to the metallic core, and the difficulty of controlling the precise disposition of the core within the composite matrix. Thus, a means of increasing blade torsional frequency is required which will not defeat the inherent advantage of the basic composite blade.
Another factor which has discouraged the introduction of composite blades into operational service on gas turbine engines is their vulnerability to what is referred to as foreign object damage (FOD). Many types of foreign objects may be entrained in the inlet of a gas turbine engine, ranging from large birds, such as seagulls, to hailstones, sand and rain. Damage from foreign objects takes two forms. Smaller objects can erode the blade material and degrade the performance of the compressor. Impact by larger objects may rupture or pierce the blades. Portions of an impacted blade can be torn loose and cause extensive secondary damage to the downstream blades and other engine components.
In this regard, the consequences of foreign object damage are greatest in the low pressure compressors, or fans, of high bypass gas turbine engines. However, these components offer the greatest potential in weight reduction due to their large tip diameters, as great as eight feet, and spans in the order of three or more feet.
The vulnerability of composite blades to foreign object damage is due to two factors. First, the lightweight matrix material employed, generally polymeric resins or metals such as aluminum, is relatively soft. Second, the high strength filaments are relatively hard and brittle. Furthermore, strain caused by the impact of a foreign object tends to travel along the filaments reinforcing the composite layers. In a cantilevered blade (for example, a blade cantilevered from the hub) the tip is unable to absorb such energy and, if the fibers communicate between the point of impact and the blade tip section, the fibers will transmit the strain to the tip section where, typically, the traveling strain waves will reinforce each other causing fracture of the blade at a location which might be substantially remote from the point of impact.
From this it became evident that a means was needed to protect the blade from FOD, thus precipitating the development of hard metallic leading edge protectors. A problem associated with such a leading edge protector is retaining it on the blade after impact due to blade bending and delamination. This, in turn, can result in secondary engine damage as the FOD protection strip is ingested through the machine or engine.
Thus, it becomes desirable to develop a blade for a high speed fluid flow machine such as a gas turbine engine compressor or fan which will transfer strain induced by foreign object impact into the hub of the blade where it can be absorbed through the blade supporting disc.